The production of reformulated gasoline satisfying new environmental regulations requires, in particular, a reduction in the concentration of olefins and/or aromatics (especially benzene), also sulphur (including mercaptans).
Catalytic cracking gasolines have high olefin contents, and the sulphur present in the gasoline pool is about 90% attributable to FCC gasoline.
Hydrotreatment of the feed sent for catalytic cracking can result in gasolines which typically contain 100 ppm of sulphur. Units for hydrotreating FCC feeds operate, however, under severe temperature and pressure conditions, which necessitates high investment.
Hydrotreatment of catalytic cracking gasolines can reduce both the sulphur content and the olefin content in the cut. However, this has the major disadvantage of causing a very large barrel octane drop in the cut, because of saturation of the olefins.
FCC gasoline hydrotreating processes have already been proposed. As an example, United States patent U.S. Pat. No. 5,290,427 describes a process consisting of fractionating the gasoline, desulphurizing the fractions and converting the gasoline fraction over a ZSM-5 zeolite.
U.S. Pat. No. 5,318,690 proposes a process including fractionation of the gasoline, sweetening the light fraction, hydrodesulphurizing the heavy fraction, then converting it over ZSM-5 and re-desulphurizing under mild conditions. That technique is based on separating the raw gasoline to obtain a light fraction which is practically free of sulphur-containing compounds other than mercaptans, so that that fraction can be treated by sweetening alone to remove the mercaptans. In this fashion, the heavy fraction contains a relatively large quantity of olefins which are partially saturated during hydrotreatment. In order to prevent this octane number drop, that patent recommends cracking over ZSM-5 to produce olefins, but this is to the detriment of the yield. Further, the olefins can be reconstituted in the presence of H.sub.2 S to form mercaptans, which has the disadvantage of requiring additional sweetening or a desulphurizing step.
In a further prior art method used by the refiner to treat the sulphur problem in gasolines, the fraction with a boiling point of at least 180.degree. C., which contains most of the sulphur-containing compounds other than mercaptans, is separated. This fraction is then downrated with LCO (light cycle oil) and is generally not upgraded, or it is used as a feed diluent.